This invention relates to planar optical circuits and more particularly to a method for lessening unwanted polarization dependence within planar waveguides of such circuits.
The invention is directed to a method for modifying the refractive index of planar optical waveguides by ultraviolet light irradiation, including but not restricted to forming an optical structure such as a Bragg or long period grating. More particularly, the novel method includes steps, which can minimize birefringence effects normally associated with writing such structures in multi-layer devices supported by a relatively thick substrate.
The sensitivity of optical waveguide fibers to light of certain wavelength and intensity has been known since the late 1970""s. It was found that the loss characteristic and refractive index of a waveguide fiber could be permanently changed by exposing the waveguide to light of a given wavelength and intensity. A publication which describes the effect and how it may be used is, xe2x80x9cLight-sensitive optical fibers and planar waveguidesxe2x80x9d, Kashyap et al., BT Techno., 1, Vol. 11, No. 2, April 1993. The publication discusses the making of light-induced reflection gratings, page 150, section 2.1, and notes that the amount of refractive index change increases as light wavelength is reduced from 600 nm to 240 nm, where the photosensitivity of the waveguide appears to peak, with the notable exception of irradiation using strong 193 nm light where the photosensitivity can be very large (as demonstrated by Malo et al. Electron. Lett. Vol. 31, p.879 (1995)).
In xe2x80x9cBragg grating formation and germanosilicate fiber photosensitivityxe2x80x9d, SPIE V. 1516, Intn""l Workshop on Photoinduced Self-Organization Effects In Optical Fiber, Meltz et al., 1991, the mechanism and magnitude of photosensitivity is discussed (page 185, first paragraph, section 1.). This publication also discusses an interferometric technique of writing gratings (pp. 185-6, section 2.) At page 189, first paragraph, a measurement of induced birefringence is presented. See also FIG. 6 of that publication.
Another publication, xe2x80x9cCharacterization of UV-induced birefringence in photosensitive Ge-doped silica optical fibersxe2x80x9d, Erdogan et al., J. Opt. Soc. Am. B/V.11, No. 10, October 1994, notes the dependence of induced birefringence on the orientation of the polarization direction of the light incident upon the waveguide fiber. In particular, data presented in the publication shows that the induced birefringence is greatest when the polarization direction is oriented perpendicular to the long axis of the fiber and least when the polarization direction is parallel to the long axis of the fiber. See FIG. 3a. and FIG. 4. of the publication.
The Erdogan et al. publication points out that the induced birefringence polarization anisotropy can be used to make such devices, xe2x80x9cas polarization mode converters and rocking filtersxe2x80x9d, page 2100, first paragraph. However, in devices using resonant propagation, xe2x80x9cthe birefringence can result in substantial polarization dependence of resonant grating properties, such as reflectivityxe2x80x9d, page 2100, first paragraph.
The Erdogan, et al., data shows that even in the configuration where the polarization direction is along the long axis of the waveguide, some birefringence is still induced in the waveguide. Comparing the curves of FIG. 3a. and FIG. 4., the non-polarization dependent induced birefringence is a factor in the range of about 4 to 12 smaller than the polarization dependent induced birefringence. However, even this smaller amount of birefringence is undesirable. A more versatile and effective grating would result from a writing method which produces a grating having minimal birefringence.
Notwithstanding, polarization effects or sensitivity from irradiating waveguides in multilayer structures exhibited as a result of disposing a relatively thin waveguide comprised of an assembly of material layers supporting low loss light propagation deposited on a thick substrate comprised of a material having different characteristics from those of the deposited layers, is significantly more evident and problematic than the effects and causes described by Erdogan et al. It is this polarization sensitivity caused by mismatching and thickness differences in layered material that is addressed by this invention.
Planar optical circuits, often termed planar lightwave circuits (PLCs) are well known and for particular applications some of which include optical gratings formed therein, such as Bragg gratings, or long period gratings. Since most signals propagating through optical fiber have an indeterminate polarization state, it is preferred that the gratings through which these signals propagate, be substantially polarization insensitive. J. Albert et al., the applicants have disclosed in a paper entitled xe2x80x9cPolarization-independent strong Bragg gratings in Planar Lightwave Circuitsxe2x80x9d Electron Lett. 34, 485-486 (1998), a method of lessening the polarization dependence or xe2x80x9cpolarization sensitivityxe2x80x9d of planar waveguides having Bragg gratings formed therein. By using an intense ArF excimer laser a refractive index change is produced and is birefringent. This birefringence is large enough and of the proper sign to compensate the inherent birefringence exhibited in most PLCs.
Notwithstanding, in the instant invention the birefringence can be controlled independently of the size of the index change. An instance where this control is particularly useful is in the path length trimming of a Mach-Zehnder interferometer that is initially polarization independent. Of course it is preferred that the trimming be nonbirefringent to maintain the polarization independence of the device. However, this invention can be used in other phased array devices, or arrayed wave guides (AWGs), requiring similar polarization insensitively in the arms of the AWG.
Planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes and therefore are known to be polarization sensitive. Stated more simply, the response of these waveguides differs for orthogonally polarized light beams. For wavelength multi/demultiplexers, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be as large as 3 nm. As WDM systems are being designed towards smaller and smaller channel spacing (from 1.6 nm to 0.8 nm or even less in the future), even a small polarization dependent wavelength shift (e.g. 0.3xcx9c0.4 nm) is of concern.
Quite surprisingly, the inventors of the instant application have discovered that the size of the beam, relative to the size of the waveguide in which a grating is to be photo-induced, greatly affects the polarization dependence of the grating or structure being written into the waveguide. For example, photo-induced birefringence occurs when irradiating a planar waveguide as described with a beam of suitable intensity having a spot size that is substantially greater than the width of the waveguide region. In some instances this birefringence offsets or compensates for the birefringence present in the planar waveguide prior to irradiation. However, most often, when writing an optical structure by photoinducing a refractive index change in the waveguide using current techniques, the amount of photo-induced birefringence cannot be accurately controlled; achieving as a desired refractive index change xcex94n does not always occur at the point where irradiation of the waveguide induces a birefringence that yields a substantially polarization insensitive device. However, by utilizing conventional techniques of irradiating with a beam sized larger than the waveguide width in combination with irradiating the waveguide with a smaller beam spot size less than or equal to the width of the waveguide channel, improved control over the polarization sensitivity of the device can be afforded. In fact, a polarization insensitive device can be manufactured. This technique is not only limited to writing gratings such as Bragg and long period gratings, but can be used to induce an index change to realize many other possible structures.
In summary it is now possible to irradiate a planar waveguide as described heretofore, with a beam having a narrow or reduced width substantially equal or less than the width of the waveguide core in combination with a beam of a substantially greater width to obviate polarization sensitivity.
It is therefore, an object of this invention, to provide a method for writing Bragg gratings and other optical structures while independently controlling the amount of birefringence induced by the irradiation. An important but not exclusive use of the invention is to substantially lessen or eliminate polarization dependence at a wavelength of interest normally associated with such structures disposed in planar waveguides.
It is a further object of the invention, to provide a photo-induced Bragg gratings having little or essentially no polarization dependence at a wavelength of interest.
The following definitions may be helpful for the understanding of this specification.
An optical waveguide grating is a periodic cyclic or a periodic variation in refractive index of the waveguide along the long axis of the waveguide.
Photo-sensitivity is an interaction between certain glass compositions and selected light wavelengths wherein incident light permanently changes the refractive index or the loss characteristics of the irradiated glass.
Side writing is a technique for forming a grating in an optical waveguide fiber wherein light is caused to form a periodic series of alternating light and dark fringes along the long axis of the waveguide. An example of such a periodic series is an interference pattern formed on the side of a waveguide fiber and along a portion of the long axis of a waveguide fiber. The periodic light intensity pattern, produced by the light interference, induces a periodic change in refractive index along a portion of the long axis of the waveguide fiber.
A planar lightwave circuit (PLC) is a waveguide having a core with a substantially square or rectangular cross section and having a cladding material of a lower refractive index surrounding the core, the whole assembly being deposited and adhering to a substrate whose thickness is substantially larger than the thickness of the waveguide layers.
PLCs are generally more polarization dependent than optical fibres having substantially cylindrically symmetric cross sections with a core centered along the axis of symmetry.
PLCs suffer both from birefringence due to the planar geometry and from material birefringence associated with fabrication processes where non equal thermo-elastic coefficient between the substrate and waveguide lead to residual strains.
It is an object of this invention to provide a planar waveguide structure having a grating or other wavelength dependent structure disposed therein, wherein the polarization sensitivity of the function of the structure is controlled to a desired value. A particular important example being that the structure be substantially polarization insensitive.
In accordance with the invention, there is provided, a method of inducing a region of modified refractive index in a planar waveguide device comprising the steps of: providing a planar waveguide comprised of layers affixed to a substrate layer, wherein at least one of
(a) an optical property,
(b) density, and
(c) thermal coefficient of expansion of the substrate differs from that of the planar waveguide layers, the planar waveguide layers being substantially thinner than the substrate layer, the planar waveguide layers having a composite thickness of t1 xcexcm; and, irradiating the waveguide with a narrow beam of light and ensuring that the beam of light incident upon the planar waveguide is restricted to a width no greater than t1 xcexcm as the beam of light impinges upon the planar waveguide.
In accordance with the invention there is further provided, a method of providing an optical structure in a planar waveguide device comprising the steps of: providing a layered structure having a composite thickness t1 which includes a thin waveguiding core layer surrounded by cladding layers, the core layer having a thickness nt1 where n less than 1.0, the layered structure being affixed to a substrate of thickness greater than mt1 where m greater than 5; and, irradiating a portion of the waveguiding core layer with a beam of light for a sufficient duration and with a sufficient intensity to permanently change the refractive index of regions within the waveguiding core layer of the portion, the beam having a spot size of less than t1.
In accordance with the invention, a method of providing an optical structure in a planar waveguide device comprises the steps of: providing a layered structure having a substrate layer of a thickness ts and a substantially thinner waveguiding core layer surrounded by a cladding having a combined thickness of t1; and, irradiating a portion of the waveguiding core layer with a beam of light for a sufficient duration and with a sufficient intensity to permanently change the refractive index of regions within the waveguiding core layer of the portion, the beam having a diameter upon the planar waveguide device wherein 95% if its power is confined to an area of less than or equal to t1.
In accordance with yet another aspect of the invention, there is provided, a method of providing an optical structure in a planar waveguide device comprising the steps of: providing a layered structure having a substrate layer of a thickness ts and a substantially thinner waveguiding core layer surrounded by a cladding having a combined thickness of t1; and, irradiating a portion of the waveguiding core layer with a beam of light for a sufficient duration and with a sufficient intensity to permanently change the refractive index of regions within the waveguiding core layer of the portion, the beam having a diameter having a non-uniform intensity pattern that varies radially, the light energy impinging upon an area of a dimension t1 or less over the waveguide core being substantially different from the light energy impinging upon other areas of the waveguide layers.
Conveniently, by employing the techniques of limiting the beam width impinging a waveguide disposed on a planar substrate in the manner that will be described, polarization sensitivity can be lessened or obviated and controlled.
Hence, the method of this invention can be useful in controlling the amount of birefringence present in a planar waveguide.